The present invention relates generally to antennas and particularly to a stub-type, monopole or sleeve-type antenna having means for decoupling it from surrounding conductive bodies.
It is frequently desired to install an antenna particularly for the transmission of high-powered signals in the UHF (300 to 3,000 MHz), VHF (30 to 300 MHz), or HF (3 to 30 MHz) bands on aircraft or ships in such a way as to minimize interference due to electromagnetic energy coupled into other electrical equipment or wiring. This problem is conventionally solved by the provision, over a required frequency range, of a very high impedance at the base of a stub, monopole, or whip antenna. This has been accomplished previously through use of a quarter-wave choke having a configuration similar to that of a coaxial transmission line. Such quarter-wave chokes are well known in the art, as exemplified, for example, by the patents to von Baeyer U.S. Pat. Nos. 2,297,512 and 2,297,513.
The principal disadvantage of such a construction is that the antenna is approximately one-quarter wavelength greater in height than the antenna without a choke. At long wave lengths, such as on the order of one to ten meters or more, difficult installation problems result. Furthermore, a quarter-wave choke causes undesirable effects on the monopole radiation pattern. However, these effects may be largely overcome by the use of a sleeve dipole antenna.
The patent to Buschbeck et al., U.S. Pat. No. 2,239,909, covers an antenna having a coaxial transmission line support. The antenna is basically a monopole antenna. Surrounding the coaxial feed line is an open circular box-like structure. This structure is excited at the inner rather than the outer periphery. The structure requires mounting so that the lower surface is exposed. Hence it could not be installed on a large conductive surface such as an airplane skin.
The patent to Carter, U.S. Pat. No. 2,359,620, discloses a short wave antenna. It consists of two opposed circular discs which are fed from the transmitter. The configuration is excited at the center and the structure is the radiating device and does not serve the purpose of isolation.
The Australian patent, No. 156,778, to Willoughby discloses an antenna structure similar to that of the Carter patent. The patent, however, describes multiple, different means of excitation. The device, again, becomes an effective radiating device or antenna rather than being used as a choke.
The patent to Lindenblad, U.S. Pat. No. 2,452,202, relates to an rf switching device which incorporated coaxial stubs and a circular resonant plate. However, the circular plate must be at least one wavelength in circumference, so that it can sustain a higher mode. This will generate an electric field which varies with azimuth.
The patent to Engle et al., U.S. Pat. No. 3,054,107, does describe an antenna making use of a radial line utilized as a choke. However, the radial line is excited at its inner radius disposed in the rotating joint of a coaxial feed line, and is not designed to utilize the characteristics provided by a very small inner radius in a radial line.